Our invention is adapted for use in a structural environment such as that described in U.S. Pat. No. 4,347,765 which comprises a transmission having a hydrokinetic torque converter and multiple-ratio, compound, planetary gearing. The relative motion of the elements of the gearing is controlled by fluid pressure operated clutches and brakes. Fluid pressure operated servos are used to selectively engage and release the clutches and brakes to effect ratio changes.
In the transmission disclosed in the '765 patent, the lowest ratio is used for accelerating from a standing start. The overall torque multiplication ratio of the transmission decreases until a final fourth ratio, which is an overdrive ratio, is obtained. The ratio changes are accomplished by an automatic control valve system. That system selectively distributes pressure from a fluid pressure pump to the fluid pressure operated servos for engaging and releasing the clutches and brakes.
The transmission pump disclosed in the '765 patent is a positive displacement gear pump located in a pump body secured to the transmission housing intermediate the hydrokinetic torque converter and the multiple ratio gearing. The torque input element of the positive displacement pump is connected drivably to the impeller of the hydrokinetic torque converter. It, in turn, drivably engages a second gear element of the positive displacement pump.
The operating speed range for the transmission of the '765 patent varies from engine idle speed to highway speed, the latter usually being a multiple of up to ten times the idling speed. When the vehicle is operating at low speeds and the engine throttle for the vehicle is advanced as the vehicle accelerates from a standing start, the torque converter impeller speed is higher than the idling speed but is still substantially lower than the final cruising speed of the engine experienced under steady state highway driving conditions.
A relatively high control circuit pressure must be maintained by the pump, especially during acceleration from a standing start with advanced throttle. It is under those conditions that the torque transfer requirements for the gearing and the fluid pressure operated servo are at a maximum, and substantial energy is required to drive the pump. This is sometimes referred to as a parasitic loss. The capacity of the pump must be designed, therefore, so that it is large enough to accommodate the maximum flow and pressure requirements when the demands on the pump are greatest. This necessarily results in excessive capacity for the pump when the vehicle is operating at high speeds under cruising conditions and the pressure and fluid flow requirements of the servos are less.
We are aware of attempts that have been made to solve the problem of excess pumping capacity in transmission pumps for automatic transmissions. One example is shown in U.S. Pat. No. 4,502,845 where a multi-stage gear pump and control valve arrangement is provided. That gear pump comprises a main drive gear that meshes with two secondary gears to define two pumping stages. The secondary pump is relieved of the burden of supplying circuit pressure by connecting the secondary pump to a low pressure sump region through a regulator valve. When the flow requirements of the pump are less, the valve responds to a pressure buildup on the downstream side of a control orifice. Upon an increase in the pump speed, the valve regulates the pressure in the discharge passage causing the discharge side of the secondary pump to become exhausted to the common sump, thereby reducing the pumping horsepower when the flow requirements are reduced.
Another prior art teaching is shown in U.S. Pat. No. 4,204,811, which discloses a compound pump with two independent gear pumps and a common driver. The pump is capable of supplying both a high pressure circuit with low flow requirements and a low pressure circuit with high flow requirements. A priority valve in a bypass flow path from the high pressure side of the pump assembly supplements the flow requirements of the low pressure load. The pump itself includes a pair of gear pumps with a common driver as in the case of the pump of U.S. Pat. No. 4,502,845.
Other prior art teachings attempt to compensate for a reduced pumping requirement by providing a bypass valve for one of a pair of pumping gears. The bypass valve acts in response to a delivery pressure signal so that there is a reduced pump power demand as one of the pumps operates against zero pressure rather than against circuit pressure. An example of this arrangement is shown in U.S. Pat. No. 4,245,964.
In prior art German patent publication DE 3402448, a compound pump has two pumping stages. The delivery rate is controlled by a flow control valve that connects the output of each pump stage to a common delivery line at low pump speeds so that the pump stages act in parallel. At high pump speeds, the valve connects the pump stages in series so that the output of one stage supplies the inlet of the other stage.
Prior art U.S. Pat. No. 3,692,432 describes a prior art compound pump wherein a regulator valve at the outlet side of one pump stage causes a pressure build-up at its inlet side, thus rendering that stage inactive while the pressure and flow requirements of the load are satisfied by the other pump stage.
Prior art U.S. Pat. No. 4,850,813 discloses a multiple stage pump system that is adapted to supply a circuit having separate regions of various pressure levels. As the flow requirements of one circuit region are satisfied, one or more of the pump stages is deactivated as its pumping function is taken over by another pump stage. Thus, each circuit region is continuously supplied with pressurized fluid with the fewest possible number of pumping stages.
French patent 904,757 describes a compound pump wherein an actuator controls a double position flow control valve that causes the output of one pump stage to be bypassed to the pump supply when the actuator responds to an increase in pump output pressure.